FIELD OF THE INVENTION
This invention relates to anticorrosive steel, more particularly to weldable coated steel with excellent corrosion resistance.
DESCRIPTION OF THE PRIOR ART
Various types of anticorrosive steel have been proposed for use under highly corrosive environments. For instance, metal plated steel such as galvanized steel or zinc alloy plated steel and weldable coated steel made by coating steel with a zinc-rich coating material called "Zincrometal" (Japanese Patent Publication No. 904/77) have been used as anticorrosive surface treated steel for automobiles. In the case of coated steel using said zinc-rich coating material, the workability, coherence and anti-corrosiveness of the coat are greatly affected by the content of zinc dust in the coating material. A high content of zinc dust is preferred for weldability, but it leads to a reduction of workability and coherence of the coat. It is known that when the content of zinc dust exceeds about 80% by weight, there occurs a phenomenon called powdering, or falling-off of the coat in powdery form, to an excess degree during press molding. On the other hand, as is well known, no desired sacrificial anticorrosive effect is obtained unless the zinc dust content exceeds 90% by weight, but such high content of zinc dust (exceeding 90% by weight) adversely affects the press moldability of coated steel.
It is thus difficult to obtain a weldable steel plate coated with a coating material containing zinc dust, which can satisfy all the requirements for workability, coherence, weldability and corrosion resistance. In an attempt to break through this problem, the present inventors previously proposed coated steel plates made by coating galvanized steel with a coating material with a low content of zinc dust (see Japanese Patent Kokoku (Post-Exam. Publication) No. 11331/79), zinc alloy-plated steel having no amount of blister on the coat, and composite-galvanized steel coated with a coating material with a low content of zinc dust (see Japanese Patent Kokai (Laid-Open) No. 189842/82 or 157995/83).
Request has risen in recent years for the longer life of car body by further improvement of corrosion resistance. However, the hitherto proposed types of surface treated steel as mentioned above were still unsatisfactory for meeting the expressed requirement for long-time corrosion resistance, such as 10-year proofing against fretting by rust, especially when such steel was used in the parts exposed to a very severe corrosive environment, for example, the inside of the car door where water and salt tends to stay pent up.
Also, when zinc dust is used as conductive powder in a coating material like Zincrometal, the formed coat is highly soft as a whole due to low hardness of zinc dust and the coated surface of steel would become too much lurbicant at the time of pressing work, causing an increased flow rate of steel into the press mold in the pressing work to make it unable to perform drawing work like cold-rolled steel, so that it becomes necessary for conducting the desired work to sharpen the beads of the mold (for example, changing the round type to square type) or to increase the blank holding force. However, the sharpened beads invite a greater chance of the coat being chipped off by the mold, and the build-up of chips would give rise to such problems as scratching and soiling of the product.
On the other hand, the coat of "weldable coated" steel contains a hard conductive powder, such as TiC, WC or stainless steel powder, mixed with soft zinc dust to afford good weldability and electrodeposition coatability to the steel, but because of the presence of hard powder in the coat, such weldable coated steel is lower in flow rate to the mold in the pressing process in comparison with normal steel and is more vulnerable to cracking in the press work. Also, if the press work of such steel is performed with a metal mold which has had no hard plating or no hardening treatment on the surface, wear of the mold is pre- cipitated.